docs: def _get_SRF_sigma(self, imt_per):
          """
          Table 8 and equation 19 of 2013 report. NB change in notation,
          2013 report calls this term 'sigma_t' but it is referred to
          here as sigma. Note that Table 8 is identical to Table 7 in
          the 2013 report.
          """
          if imt_per < 0.6:
              srf = 0.8
          elif 0.6 <= imt_per < 1:
              srf = self._interp_function(0.7, 0.8, 1, 0.6, imt_per)
          elif 1 <= imt_per <= 10:
              srf = self._interp_function(0.6, 0.7, 10, 1, imt_per)
          else:
              srf = 1
  
          return srf